Electrostatic coating apparatus



Aug. 22, 1967 M. A. R. POINT 3,336,903

ELECTROSTATIC COATING APPARATUS Filed April 21, 1964 2 Sheets-Sheet l Aug. 22, 1967 M. A. R. POINT 3,336,903

I ELECTROSTATIC COATING APPARATUS Filed April 21, 1964 2 Sheets-Sheet 2 Fig.3

United States Patent f This invention relates to electrostatic coating apparatus and more particularly to such apparatus for applying a finely divided coating substance to a surface of an article to be coated.

There has been developed a method and apparatus for coating articles with a fluidized composition which is ex tremely rapid in action and highly eificient. One such method and apparatus is disclosed, for example, in Bar ford et a1. copending US. application Ser. No. 287,554, filed June 13, 1963, now patent No. 3,248,253. Methods and apparatus of this type customarily include a stationary vat or other container for the particles of coating material which is supplied with compressed air to form a fluidized bed or cloud of the particles. The particles are electrically charged to a high DC. potential by a series of charging electrodes which project into the cloud. The articles to be coated are presented in succession in the vicinity of the electrically charged cloud, and the indivi dual particles are electrostatically attracted to the surface of each article to form a uniform and smooth coating.

In some cases, a portable electrostatic spray gun is substituted for the stationary container. A cloud of the particles of coating material is projected from the discharge nozzle of the gun and is electrostatically charged to produce an attraction between the particles and the surface of the article being coated. The thus charged particles settle uniformly over the article to provide the desired coating.

Electrostatic coating methods and apparatus of the foregoing type exhibit numerous important advantages over more conventional coating techniques. These advantages include a higher uniformity in the depth of the coating, substantially reduced losses of coating material and an improved bond between the individual particles of material and the surface of the article. Accordingly, and particularly in the light of the comparatively recent development of compact electrostatic generators for producing the needed voltages, such processes and apparatus have gained Wide acceptance.

Heretofore, in the use of electrostatic coating equipment of the type discussed above, difliculties have been encountered in providing adequate control over the spatial extent of the electrostatically charged cloud of particles. As an illustration, and this has been of special moment in equipment employing a stationary vat or con-' tainer, it often was difficult to provide adequate control over the various operating parameters to insure that the cloud of particles rose to a suflicient height within the container to be freely attractable by the Work and yet not so high that an appreciable quantity of particles spilled from the container. In addition, it has been found that the cloud formed by many such prior apparatus exhibited a tendency to wander, particularly under the influence of stray fields, for example, with the result that the maximum advantages of the apparatus were not utilized to their fullest extent.

One general object of this invention, therefore, is to provide new and improved electrostatic coating apparatus for applying a finely divided coating substance to a surface of an article to be coated.

More specifically, it is an object of this invention to provide such apparatus which enables the exercise of a 3,336,903 Patented Aug. 22, 1967 precise control over the spatial extent of a floating cloud of electrically charged particles.

Another object of this invention is to provide electrostatic coating apparatus of the character indicated in which the adverse effects of stray electrical fields are substantially reduced.

A further object of the invention is to provide electrostatic coating apparatus utilizing comparatively simple components which is economical to manufacture and thoroughly reliable in operation.

In accordance with several preferred embodiments of the invention, there is provided electrostatic coating apparatus which includes a supply of fluidizing gas for forming a floating cloud of the finely divided coating material to be applied to the surface of the article. One or more charging electrodes are positioned Within the cloud and are supplied :with a high DC. potential, say, one hundred kilovolts, to electrically charge the individual particles to a corresponding potential. An electrostatic field is thereby set up between the particles and the surface of the article to carry the particles toward the article.

In accordance with one feature of the invention, a control electrode is arranged in spaced juxtaposition with the charging electrode and is electrically grounded or otherwise connected to a potential different from the potential of the charged particles in the cloud of coating material. Individual charged particles adjacent the control electrode are discharged thereby to prevent the spreading of the cloud beyond predetermined well defined limits. The arrangement is such that any tendency of the cloud to extend beyond these limits is avoided, and the adverse effects resulting from extraneous electrical fields are substantially reduced.

The foregoing and other objects and advantages of the invention will appear more clearly and fully from a reading of the following description, given by way of example, of various preferred embodiments, reference being made to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic view, with certain parts shown in section, of electrostatic coating apparatus in accordance with one illustrative embodiment of the invention;

FIGURE 2 is a simplified perspective view, partly broken away and in section,

of electrostatic coating apparatus in accordance with another illustrative embodienclosed chamber beneath the bottom to the main bod} portion of the container.

The false bottom 2 supports a mass 3 of finely divided coating material which is to be applied to the articles (not shown in FIGURE 1) to be coated. These articles customarily are advanced in succession (as by an overhead conveyor, for example) to a position immediately above the container 1. The coating material illustratively comprises a synthetic resin, powder enamel, metallic powder or other finely divided substance which is fluidized to form a cloud of particles within the container. One end of a fluidizing conduit 4 communicates with the chamber beneath the false bottom 2, while the other end is supplied with compressed air or other fluidizing gas under in accordance with still another pressure. As the air is introduced from the conduit 4 into the chamber, it passes in an upward direction through the pores or perforations in the bottom 2 and maintains the particles 3 in a state of continuous agitation and turbulence to form the cloud. The individual particles in the cloud describe random trajectories somewhat in the manner of the molecules of a fluid, but with the cloud as a whole remaining substantially stationary within the container. The level of the upper boundary of the cloud is of course substantially above the level of the coating particles when in a static or non-fluidized condition.

A series of upstanding point electrodes or ionizers 6 are positioned adjacent the upper surface of the false bottom 2 in juxtaposition with the cloud 3. These electrodes are affixed to a horizontal supporting rack 10 immediately above the bottom 2 with the points of the electrodes extending upwardly into the fluidized cloud. The electrodes 6 advantageously are arranged in spaced relationship with each other in a planar array throughout the horizontal extent of the cloud.

In the embodiment illustrated in FIGURE 1, the electrodes 6 are of electrically conductive material, although in other advantageous arrangements resistive or semi-conductive electrodes are employed with good efi'ect. The supporting rack 10 is electrically conductive and serves to connect the individual electrodes to an insulated conductor 11 leading to the negative terminal of a high DC. voltage generator 7. The generator 7 is of conventional construction and is arranged to develop an output potential which illustratively is of the order of about one hundred kilovolts.

The high D.C. potential from the generator 7 is applied to the point electrodes 6 to charge the individual particles 3 of coating material in the cloud to a corresponding potential. The article to be coated, on the other hand, is at a diiferent potential and customarily is connected to ground. An electrical field is thereby established between the particles and the article, and the particles are guided by electrostatic attraction along lines of force toward the surface of the article to form a uniform and smooth coating. The point electrodes 6 act both through direct con tact with the agitated particles and through the intermediacy of ionized air molecules to impart to substantially all of the particles in the container a high DC. potential corresponding to the output potential of the generator 7. The charged particles are rapidly and efficiently applied to the surface of the article and cling firmly thereto as a result of the electrostatic attraction.

The structure described thus far is merely illustrative of a known combination of electrostatic coating components. It will be readily apparent that the upper level of the cloud of particles 3 should be sufficiently close to the article above the container 1 to enable the electrostatic deposition of the particles thereon. On the other hand, particularly prior to the time the article approaches the container, the elevation of the cloud should be maintained beneath the rim of the container to avoid spilling or other discharge of the particles. Although the upper level of the cloud of course is determined in part by the rate of flow and velocity of the fluidizing gas delivered through the conduit 4, electrical factors also intervene. Because the individual particles are charged to the same high DC. potential, they tend to repel One another. Moreover, particularly when coating comparatively small articles or prior to the time the article is in position above the container, the presence of stray or parasitic electrical fields adversely affect the stability of the cloud. These fields for the most part are set up between the electrodes 6 and surrounding objects at ground potential and produce a tendency for particles in the upper portion of the cloud to wander or spill from the container.

In the embodiment illustrated in FIGURE 1, there is provided a set of spaced-apart control electrodes 8 which are effective to limit the spatial extent of the cloud of coating particles within the container 1. Each of the electrodes 8 is in the form of a point and is generally similar to one of the charging electrodes 6. The electrodes 8 are mounted on the inner side walls of the container 1 at a predetermined height above the electrodes 6 and in spaced relationship with the article to be coated. The pointed ends of the electrodes 8 project into the container in a generally horizontal direction and are at ground or other potential substantially similar to that of the article.

Upon the introduction of compressed air through the conduit 4 and upwardly through the container wall 2, the particles of coating material form a cloud and are electrically charged by the charging electrodes 6. As individual particles approach the control electrodes 8, they are discharged both by direct contact with the electrodes and particularly through the ionization of air molecules to create ions of a polarity opposite to tthat of the particles. The number and position of the electrodes 8 are such that the height of the fluidized cloud of particles is controlled with a good degree of accuracy both in the presence of a grounded article above the container 1 and in the absence of such article. In addition, the upper level of the cloud is substantially independent of the size of the articles, and even comparatively small articles are coated without undesirable spilling of particles from the container. The electrodes 8 in effect establish a controlling electrostatic field around the upper marginal periphery of the container 1 which improves the stability of the cloud and renders it less sensitive to undesired extraneous fields.

During the coating operation, the container 1 advantageously is vibrated by means of a vibrator device shown schematically at 5. The vibrator 5 is mounted on the horizontal bottom wall of the container and serves to shake off individual particles from the fluidized cloud which settle on the control electrodes 8. The thus removed particles drop into the container where they are recycled in the cloud and are again charged to a high D.C. potential by the electrodes 6. The vibrator 5 also improves the fiuidizing action.

In the modification shown in FIGURE 2, the mass 3 of coating particles within the container 1 is fluidized through the use of compressed air to form a floating cloud of charged particles in a manner similar to that described above. A pair of control electrodes in the form of fine wire loops 8 are disposed in horizontal planes on the inner walls of the container 1 immediately beneath the containers upper periphery. These loops extend throughout upper marginal areas of the cloud and are fabricated from stainless steel wire which illustratively has a diameter of about 0.1 or 0.2 mm. The loops 8' are vertically spaced from each other and are supported from the inner wall of the container by suitable supports 9 of dielectric material. Both of the loops are in spaced relationship with the charging electrodes (not visible in FIGURE 2) and are electrically connected to ground, as is the article to be coated.

As individual charged particles in the cloud approach the wire loops 8', the particles are discharged and return to the main body portion of the cloud within the container 1. The spatial extent of the cloud is thereby limited to substantially eliminate any possibility of the particles spilling from the container or being attracted by extraneous electrical fields.

In the embodiments of FIGURES 1 and 2, the equipment for forming the fluidized cloud of particles and applying electrostatic charges thereto includes a stationary container supplied with gas under pressure and a series of point electrodes. In other good arrangements, the particles are fluidized and charged through the use of portable equipment such as an electrostatic spray gun. As an illustration, in FIGURE 3 there is shown a spray gun 21 which is arranged to apply liquid paint or other coating material to an article 25. This article illustratively is in the form of a flat panel of metal or plastic. The spray gun 21 is provided with a nozzle 22 operatively associated therewith which comprises a charging electrode of conductive or semiconductive material. The nozzle 22 is electrically connected to the insulated negative tenminal of a high voltage D.C. electrostatic generator 23, the other terminal of which is connected to ground. The finely divided coating material is delivered to the gun 21 in a stream of air or other gas under pressure from a flexible hose 24 and is directed through the nOZZle 22 toward the surface of the article 25. For a more detailed discussion of representative spray guns which may be employed in connection with the embodiment of FIGURE 3, reference may be had, for example, to Felici et al. copending US. application Ser. No. 274,268, filed Apr. 19, 19 now Patent No. 3,279,529.

The article 25 is suspended from a monorail conveyor or other suitable overhead support, shown schematically at 29, and is electrically connected to ground. The fluidized particles of coating substance are charged to a high DC. potential corresponding to that of the generator 23 as they issue from the nozzle 22. The particles form a cloud adjacent the nozzle 22 which is charged substantially entirely throughout its spatial extent. This cloud proceeds to float at a relatively slow velocity along the lines of force of the electric field present between the nozzle 22 and the article 25.

To reduce the possibility of any straying or wandering of the particles moving between the nozzle 22 and the article 25, such as might result because of extraneous electrostatic fields, for example, there are provided a series of control electrodes 26 in spaced juxtaposition with the nozzle. The electrodes 26 are arrayed adjacent the side of the article 25 remote from that facing the spray gun 22 and are in the form of fine electrically conductive wires, in this case four widely spaced vertical wires, connected to ground or other potential substantially similar to that of the article. The electrodes 26 are carried by a vibrator device 27 which is effective to facilitate the removal of powder particles which settle on the individual wires. Suitable collecting means (not shown) may be provided beneath the electrodes 26 for collecting the particles from the wires and recycling them through the flexible hose 24. A screen 28 of electrically non-conductive material advantageously is positioned behind the wires 26 in spaced relationship therewith to reflect any particles passing through the wires.

In the absence of an article on the support 29, or in cases in which the size of the article is comparatively small, individual charged particles of coating material approach the grounded control electrodes 26. These particles are discharged by direct contact and by the surrounding ionized air molecules. The spatial extent of the cloud of particles is controlled such that the particles are confined by the control electrodes to provide a substantial improvement in the clouds stability.

It will be understood that various modifications other than those shown and described may be utilized in connection with the invention. The control electrodes may assume forms other than those illustrated and may be arrayed in various positions. As an illustration, in the spray gun arrangement of FIGURE 3, the electrodes 26 may be positioned along opposite sides of the spraying station or at other locations. In several advantageous embodiments, to facilitate effective ionization the active portions of the control electrodes preferably are of small radius, that is, in the form of sharp points, thin wires or the like. Other, larger radius portions of the control electrodes may if necessary be covered with insulation. The invention in many respects is applicable to the coating of articles with finely divided liquid droplets, e.g., paint, as well as to the coating of articles with fine powder particles.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.

What is claimed is:

1. Apparatus for coating an article with a finely divided coating substance comprising, in combination, means for forming a fluidized bed of the finely divided coating substance adjacent a surface of the article to be coated, charging electrode means in juxtaposition with said fluidized bed for applying a first electrical. potential to the individual particles of coating substance therein, means for connecting said article to a second electrical potential, to establish an electrostatic field between said particles and the surface of said article, the thus charged particles being guided by electrostatic attraction toward said surface, and control electrode means at a potential different from said first potential and positioned in spaced relation with said charging electrode means for controlling the spatial extent of said fluidized bed.

2. Apparatus for coating the surface of an article with a finely divided coating substance comprising, in combination, means for forming a fluidized bed of the finely divided coating substance adjacent the surface of said article, charging electrode means supplied with a high DC. potential, said charging electrode means being positioned in close proximity with said fluidized bed to charge the individual particles of coating substance therein to said high DC. potential, to establish an electrostatic field between said particles and the surface of said article whereby charged particles from said bed are guided by electrostatic attraction toward said surface, and control electrode means connected to a potential substantially different from said high DC. potential and positioned around the periphery of the fluidized bed in spaced relation with said charging electrode means for controlling the spatial extent of said fluidized bed.

3. Apparatus for coating an article with a finely divided coating substance comprising, in combination, a container for the coating substance adjacent a surface of the article to be coated, means for directing fluid under pressure into said container to form a fluidized bed of the coating substance, charging electrode means positioned within said container in juxtaposition with said fluidized bed for applying a first electrical potential to the individual particles of coating substance therein, the thus charged particles being guided by electrostatic attraction toward said surface, and control electrode means at a potential different from said first potential and positioned in spaced relation with said charging electrode means for controlling the spatial extent of said fluidized bed.

4. Electrostatic coating apparatus for applying a finely divided coating substance to a surface of an article, said apparatus comprising, in combination, a container for the coating substance, means for delivering fluidizing gas under pressure into said container, said fluidizing gas forming a fluidized bed of finely divided particles of said coating substance, charging electrode means supplied with a high DC. potential relative to the potential of the article to be coated and positioned within said container for charging said fluidized bed to said high DC. potential, means for connecting said article to ground potential, to establish an electrostatic field between said particles and the surface of said article, whereby particles from said bed are guided by electrostatic attraction toward the surface of said article, and control electrode means connected to a potential substantially similar to that. of said article and positioned above said charging electrode means in spaced relation therewith for controlling the vertical extent of said fluidized bed.

5. Electrostatic coating apparatus for applying a finely divided coating substance to a surface of an article, said apparatus comprising, in combination, a container for the coating substance having a foram'inous false bottom, said container defining a substantially enclosed chamber beneath said false bottom, means for delivering fluidizing gas under pressure into said chamber, said fluidizing gas being discharged from said chamber through said false bottom to form a fluidized bed of finely divided particles of said coating substance, charging electrode means supplied with a high DC. potential relative to the potential of the article to be coated and positioned adjacent said fluidized .bed for charging the particles therein to said high DC. potential, whereby particles from said bed are guided by electrostatic attraction toward the surface of said article, and control electrode means connected to a potential substantially similar to that of said article and positioned in spaced relation with said charging electrode means, said control electrode means being supported by the inner wall of said container and extending around the fluidized bed for controlling the spatial extent of said fluidized bed.

6. Electrostatic coating apparatus of the character set forth in claim 5, in which said control electrode means and said article are connected to ground potential.

7. Electrostatic coating apparatus for applying a finely divided coating substance to a surface of an article, said apparatus comprising, in combination, a container for the coating substance having a foraminous false bottom, said container defining a substantially enclosed chamber beneath said false bottom, means for delivering fluidizing gas under pressure into said chamber, said fluidizing gas being discharged from said chamber through said false bottom to form a cloud of finely divided particles of said coating substance, charging electrode means including a first set of spaced-apart point electrodes supplied with a high D.C. potential relative to the potential of the article to be coated, said first set of electrodes being positioned adjacent the upper surface of said false bottom for charging said cloud substantially entirely throughout the extent thereof to said high DC. potential, whereby particles from said cloud are guided by electrostatic atttraction toward the surface of said article, and control electrode means including a second set of spacedapart point electrodes connected to a potential substantially similar to that of said article, said second set of electrodes being supported by the inner wall of said container in spaced relation with said first set of electrodes for controlling the vertical extent of said cloud.

8. Electrostatic coating apparatus for applying a finely divided coating substance to a surface of an article, said apparatus comprising, in combination, a container for the coating substance having a foraminous false bottom, said container defining a substantially enclosed chamber beneath said false bottom, means for delivering fluidized gas under pressure into said chamber, said fluidizing gas being discharged from said chamber through said false bottom to form a cloud of finely divided particles of said coating substance, charging electrode means supplied with a high DC. potential relative to the potential of the article to be coated and positioned adjacent the upper surface of said false bottom for charging the cloud substantially entirely throughout the extent thereof to said high DC potential, whereby particles from said cloud are guided by electrostatic attraction toward the surface of said article, and control electrode means including at least one wire loop connected to a potential substantially similar to that of said article for controlling the vertical extent of said cloud, said loop being supported by the inner wall of said container in spaced relation with said charging electrode means and extending throughout marginal areas of said cloud.

References Cited UNITED STATES PATENTS 2,698,814 1/1955 Ransburg 118624 X 2,711,155 5/1955 Starkey 118-624 2,773,472 12/1956 Lamm 118-624 2,955,565 10/1960 Schotland 1187 3,004,861 10/1961 Davis 11718 3,054,697 9/1962 Irland et al. 118622 X 3,059,613 10/ 1962 Nakaya 118624 3,248,253 4/1966 Barford et a1. 11717 3,254,625 6/1966 Armstrong 118612 3,255,730 6/1966 Grohl 118-624 CHARLES A. WILLMUTH, Primary Examiner.

PETER FELDMAN, Assistant Examiner. 

1. APPARATUS FOR COATING ARTICLE WITH A FINELY DIVIDED COATING SUBSTANCE COMPRISING, IN COMBINATION, MEANS FOR FORMING A FLUIDIZED BED OF THE FINELY DIVIDED COATING SUBSTANCE ADJACENT A SURFACE OF THE ARTICLE TO BE COATED, CHARGING ELECTRODE MEANS IN JUXTAPOSITION WITH SAID FLUIDIZED BED FOR APPLYING A FIRST ELECTRICAL POTENTIAL TO THE INDIVIDUAL PARTICLES OF COATING SUBSTANCE THEREIN, MEANS FOR CONNECTING SAID ARTICLE TO SECOND ELECTRICAL POTENTIAL, TO ESTABLISH AN ELECTROSTATIC FIELD BETWEEN SAID PARTICLES AND THE SURFACE OF SAID ARTICLE, THE THUS CHARGED PARTICLES BEING GUIDED BY ELECTROSTATIC ATTRACTION TOWARD SAID SURFACE, AND CONTROL ELECTRODE MEANS AT A POTENTIAL DIFFERENT FROM SAID FIRST POTENTIAL AND POSITIONED IN SPACED RELATION WITH SAID CHARGING ELECTRODE MEANS FOR CONTROLLING THE SPATIAL EXTENT OF SAID FLUIDIZED BED. 